Signal-transmission system



0% 1945 A. v. LOUGHREN SIGNAL TRANSMISSION SYSTEM Filed Dec. 7, 1944 3 Sheets-Sheet 1 E E Wm 2925. ccmmwmewcuc.

m EwCP INVENTOR. ARTHUR V. LOU GHREN ATTORNEY A. v. LOUGHREN 2,388,4IOI

SIGNAL TRANSMISSION SYSTEM Filed Dec. 7, 1944 3 Sheets-Sheet 2 ATTO NEY atenied ct. 3, 145

SKGNAIL-SWSSION SYSTEM l Arthur V. Loughren,

Great Neck, N, Y., assignor to llilazeltine ilorporation, Washington, D. 0., a corporation of Delaware Application December 7, 19%, Serial No. 567,049 I 17 Claims.

The present invention relates to signal transmission systems and, particularly, to such systems wherein a given number of signal sources are accommodated by a lesser number of signal transmission channels by the method of time sharing, thus to reduce the time during which any transmission channel remains idle. While the invention isof general application, it is particularly suited for speech transmission systems and will be described in that connection.

In communication over long distances, the number of telephone lines or other transmission channels available to a given service is often much more limited than the number of persons or subscribers who may wish to communicate at a given time. Since the peak demand made on a communication system of this nature occurs only during one or two relatively short periods of the day, and may be greatest only during a certain season of the year, it is not economical to provide a sufficient number of transmission channels always to be able to allocate an individual channel to each person or subscriber wishing to communicate over the system. Rather, a, limited number of transmission channels are provided and the persons wishing to use these channels must then do so by awaiting their turn, Or some form of rapid time-sharing arrangement is utilized. Statistical studies indicate that in ordinary dialogue the actual time required by the spoken words of one person is approximately V of the entire conversational time.

Perhaps the majority of prior time-sharing arrangements, for utilizing transmission channels more emciently, employ some form of rotational distribution of the available transmission channels between a large number of active subscribers. A transmission channel is assigned to any one subscriber only during the actual conversational period of that subscriber Or perhaps only during the actual word intervals of his conversation. By the term "word interval as used herein is meant the duration of one or a succession of syllables no one of which is separated from another by an interval as great as that between successive words of a conversation. This term may include the hang-over time, an expression commonly used in telephone practice in connection with telephone systems including voice-operated devices, which is the period of time that a voiceoperated device remains, due to its own operating characteristics, in operated position after the operating impulse has been removed. A small amount of "hang-over time insures that weak endings of syllables which by themselves are not capable of holding a voice-operated relay in operated position will not be lost due to a premature opening of the relay. The hang-over time may be increased or decreased in duration as the type of systems demands. Of the prior time-sharing systems mentioned, the one wherein a transmission channel is assigned to a given subscriber for the entire period of a sentence or group of sentences without intervening-pauses has the disadvantage that.it does not eifect the greatest utilization of a given transmission channel since the latter must necessarily remain idle during the intervals between words of the conversation. The method of time sharing by word intervals efiects somewhat higher emcieency, but has the disadvantage that it does not efiect the most desirable distribution of the available channels between the active subscribers.

During the past decade, the tendency in multichannel communication systems has been toward the utilization of a single conductive circuit designed to translate a relatively wide frequency band and to divide this band into a plurality of discrete communication channels of lesser band width. The speech or telegraphic signals to be transmitted are carried as modulation components of carrier waves having individual frequencies such that desired componentsmay be separated by band-pass selectors and individually applied to the plurality of available transmission channels. Since the cost of the transmission circuit varies with the band width which it must have to accommodate the desired number of transmission channels, it is desirable that its band width be minimized for a given number of transmission channels or, conversely, that the greatest number of transmission channels be accommodated in a given band width.

It is an object of the present invention to provide a new and improved signal-transmission systerm which avoids one or more of the limitations and disadvantages of prior systems of the type described.

It is a further object of the invention to provide a new and improved signal-transmission system which substantially increases the efiiciency of communication systems by decreasing the time the transmission channels of the system remain idle.

It is an additional object of the invention to provide a new and improved signal-transmission system of the carrier-wave type which possesses the desirable characteristics previously men tioned.

In accordance with a feature of the invention,

a signal-transmission system comprises a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, and means coupling the sources to the channels for temporarily assigning all of the sources when inactive to one of the channels. The system includes means responsive to a predetermined operating characteristic 01 active ones of the sources for controlling the coupling means to maintain the lowest-order active source assigned to the aforesaid one channel and higher order active sources assigned in order individually to consecutive channels proximate to the aforesaid one channel.

In accordance with a particular form of the invention, a signal-transmission system of the type described includes means responsive to a predetermined operating characteristic of active ones of the sources for coupling the active sources during the active signal interval of each selectively to the transmission channels, and means for conditioning the system to assign a plurality of the active sources to each such channel including means responsive to the band width of the signal or" each active source for controlling in accordance therewith, during the aforesaid each active signal interval, the width of the frequency spectrum assigned to each such active source.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring now to the drawings, Fig. 1 illustrates schematically the general arrangement of a signal-transmission system embodying the present invention; Fig. 2 is a circuit diagram, partly schematic, of several transmitting stations arranged in a system embodying the invention; and Fig. 3 is a circuit diagram, partly schematic, of several receiving stations arranged in the system of the invention.

Referring particularly to Fig. 1, there is shown the general arrangement of a plurality of transmitting stations S1 to S10, inclusive, a plurality of corresponding receiving stations R1 to R10, inclusive, and a plurality of interconnecting transmission channels T1 to T6, inclusive, in a signal transmission system embodying the present invention. The transmitting stations S1 to S10, inclusive, and corresponding receiving stations R1 to R10, inclusive, ma be considered as arranged in consecutive order and it will be noted that the number of transmitting and receiving stations exceeds the number of transmission channels provided in the system. Statistical data indicates that, in practice, the number of transmitting stations may exceed the number of transmission channels in the ratio of 3:1 without too greatly impairing the overall operation of the system. In the ensuing discussion of the Fig. 1

group including stations S1 to S5, inclusive, and a second group including stations Se to S10, inclusive. The system of the present invention temporarily assigns all inactive stations of each group to individual end transmission channels T1 and T0. As the stations become active, the assignment of the lowest-order active station to the channel T1 is maintained while the higherorder active stations are assigned in order individually to consecutive channels T2, T3, etc., proximate to the channel T1. The terms "lowerorder and higher-order are used in the present specification and claims simply to denote the order in which the stations, or the speech-signal sources thereof, have priority of assignment to the available channels, the lower-order stations having priority of assignment over a high-order station.

Assume for purposes of illustration that the stations of the first group S1 to S5, inclusive, have increasing order from the station S1 to the station S5. Further assume that the stations S3 and S are active at a given moment. As indicated by the solid-line arrows, the active stations & and S5 are so assigned to the available channels that the assignment of the lowest-order active station 83 to the first channel T1 is maintained while the higher-order active station S5 is assigned to the second channel T2. If now one of the even lower-order stations S1 or S2 were to become active, this station would immediately be assigned the first channel T1 and the active stations S3 and S5 would be shifted to the respective channels T2 and Ta. Alternatively, should the station S4 become active, the fact that it is of higher order than the station S: but of lower order than the station S5 would have the result that it would not disturb the assignment of the station S: to the first channel T1, but would rather, by virtue of its priority, cause the station S5 to be newly assigned to the channel T3 to enable the assignment of the station S4 to the secsystem, reference is made only to the transmitting stations S1 to S10, inclusive, but it will be understood that provision is made, as will hereinafter be described in detail, by which the assignment of one of the transmitting stations to a given transmission channel effects the automatic assignment of the corresponding receiving station to the same channel.

For reasons which will become more fully apparent during the description of the Fig. 2 transmitter system, the transmitting stations preferably are divided into two groups having equal numbers of stations in each, for example, a first ond channel T2.

The assignment of the stations to the transmission channels in this manner is efiected by word-actuated relay devices and is so rapid that a word of the station S0, for example, may very well have one or two of its syllables transmitted over the channel T2 and its remaining syllables transmitted over the channel T3. As previously mentioned, provision is made for the receiving stations R1 to R10, inclusive, automatically to follow the assignment of their corresponding transmitting stations so that one of the receiving stations will always receive a complete word originating at a corresponding transmitting station even though the changing assignments of the transmitting stations occur with such rapidity that portions of a word are transmitted over several transmission channels.

The second group of stations S6 to S10, inclusive, likewise are assignable in order to available transmission channels, but in this case all of the stations when inactive are assigned to the end transmission channel To. In this case also, the order of the stations, or order of priority of assignment, is considered with relation to station S10 so that the station Se is the highest-order station of the group. Thus, as illustrated in Fig. 1 by way of example, assume that the stations S10, S11, and S7 are active at a given moment. BY priority, theSe active stations are assigned to the respective transmission channels, T5, T5, and T4 as indicated by the solid-line arrows. If now the station S0 becomes active, it is of higher asaaooi order than the station S but of lower order than the stations Se and S1 so that it takes priority on the channel T5 and shafts the stations Se and S7 to the respective channel T4 and T3, as indicated by the broken-line arrows.

The two groups of stations S1 to S5, inclusive, and So to S10, inclusive, are not restricted to a predetermined number of the channels T1 to To, inclusive, but may use as many of the channels as are needed. Thus, it may happen that during one moment of time the first group of stations may use all of the channels T1 to T5, inclusive, while a moment later the second group of stations may all become active and be assigned to all of the channels T2 to To, inclusive. If a sumcient number of channels are provided for a given number of stations, for example a ratio of channels to stations of the order of /2 to /3, the probability of one channel being thus momentarily assigned to two stations is in practice rather remote since so much of the time required for one person to carry on a two-way conversation is "idle time." However, should it happen that two stations are momentarily assigned to a common transmission channel, the assignment can only occur for words simultaneously spoken by both stations so that neither of the two receiving stations concerned is able to receive an intelligible unauthorized word and the net effect will be the same as though a momentary burst of noise disturbance had appeared on that channel of the transmission system. On the other hand, it will be apparent from the foregoing brief description of the system that the end transmission channels T1, T2, T5 and To have extremely high efiiciency since they are used almost continuously and the center channels T3 and T4 are operated with very good efficiency since they may be used by the transmitting stations of either group and thus will be in use a great majority of the time.

An additional important feature of the transmission system of the present invention, presently to be described in greater detail, concerns the assignment of any active station to only a sumcient portion of an available transmission channel adequately to transmit the signal of the station. This permits the momentar assignment of two or more stations, without interference one with the other, to a single transmission channel, whereby the number of available transmission channels may be eifectively increased in substantial degree. Thus, assuming that a transmission channel has a band width of 3,000 cycles, a lowerorder station which requires only a band width of 1,500 cycles for the transmission of its signal will be'assigned /2 of a transmission channel to leave the other half of the channel available for use by a second station of higher order. It may therefore happen that the signal of a given transmitting station is, at a given moment, transmitted over portions of two adjacent channels to its corresponding receiving station, the lower frequencies of the signal being transmitted over one channel and the higher frequencies over another. This, however, will not impair the fidelity of the transmitted signal since the corresponding receiving station similarly is assigned to corresponding portions of the two adjacent channels and thus receives both the lower and higher frequencies of the transmitted signal.

It will be understood, of course, that the use of ten stations and six transmission channels in Fig. 1 is only for purposes of illustration and that, in practice, a system embodying the present invention may have as many such stations as can conveniently be utilized with a given large number of transmission channels.

In the following detailed description of a system embodying the present invention, the transmission channels are described as having a band width of 3 kilocycies and as being contiguous over a predetermined range of the frequency spectrum, two active stations being assigned to equal portions of the same channel where a lower-order active source requires at a given moment only V of the channel band width. It is to be understood that this view of the channel arrangement is one selected simply for purposes of convenience in describing the invention and its operation and that an alternative view of the channel arrangement is one in which each channel of a contiguous arrangement of channels has a 1.5 kilocycle bandwidth with an active station assigned to two adjacent channels where the momentary frequency components of the speech signal of such active station require a 3-kilocycle band width for transmission thereof. Yet another manner of considering the channel arrangement is one in which each channel occupies a frequency region the width of which is determined by the momentary demand of the signal of an active source coupled to the channel and the position of which in the frequency spectrum is determined by the number of active lowercrder sources at a particular moment. Consequently, it is intended that the terminology of the appended claims relating to the transmission channels, their arrangement, and the order in which active sources are coupled thereto is intended to cover arrangements of any of the types Just described.

Fig. 2 is a circuit diagram, partly schematic, representing several complete transmitting sta tions S1 to S3, inclusive, arranged in a signal 40 transmission system embodying the present invention. The three stations shown include in large part similar circuit elements in a similar circuit arrangement. Hence, to simplify the de scription of the invention, elements of stations 32 and S3 which correspond to similar elements of station S1 are designated by similar reference numerals primed for station S2 and double primed for station S3. Station S1 includes a speechsignal source it. Since the stations S1 to S3, inclusive, may be considered as arranged in consecutive order as previously mentioned, the signal sources l0, l0 and it" thereof may also be considered as arranged in consecutive order in similar manner. The signal-transmission system of the present invention also includes, as earlier mentioned, a plurality of transmission channels. In particular, the system includes transmission means, comprising for example a coaxial transmission line H, having a frequency spectrum pro-r viding such channels in consecutive order.

The system also includes means coupling the sources l0, l0 and E0" to the channels it for temporarily assigning all of the sources when inactive to one of the channels, preferably an end channel as previously described. In particular, this means comprises heterodyne means for heterodyning the signal appearing in any single active one of the sources l0, l0 and 00" to one channel provided by the frequency spectrum of the transmission line H. In greater particularity, this means for station S1 comprises a pair of band-pass selectors it, it having input circuits coupled to the output circuit of the signal source mon input circuit of a modulator is. For speech cuits of the modulators l5 and 85" of the other stations, an oscillator 26 which may have a frequency, for example, of 92 kilocycles. The frequency values and translation band widths assigned to the oscillator l6 and to other subsequent elements of this and others of the stations are indicated on the drawings. These frequency values are illustrative values suitable for three stations of a total of twenty or so such stations. There is coupled between the output circuit of the modu lator i5 and transmission channel ii, in the order named, a band-pass selector H which translates a frequency band from 92 to 95 kilocycles, a modulator it, and a band-pass selector idwhich translates a frequency band from 10 to 46 kilocycles. An oscillator 2@, having a frequency of 82 kilocycles, is coupled to an input circuit of the modulator it. In similar fashion, the second station S2 has an oscillator 29' coupled to an input circuit of the modulator i8, but in this case the oscillator is of the controllable frequency type and may have any of several selected frequencies between 79 and 82 kilocycles as will presently be explained in greater detail. In station 83, there is coupled to an input circuit of the modulator is" an oscillator 2t, also of the controllable frequency type, which may have any of several selected frequencies within the range from 76 to 82 kilocycles.

The signal-translating system also includes means responsive to a predetermined operating characteristic of active ones of the sources to, ill and ill" during Word intervals thereof for controlling the coupling means last described to maintain, from moment to moment, the lowest-order active source assigned to one channel and higherorder active sources assigned in order individually to consecutive channels proximate to the one channel. This means essentially comprising a control system for the heterodyne means described. This control system heterodynes the signal of the lowest-order active source l0, ID or In" to the aforementioned one channel and heterodynes the signals of higher-order active sources individually to consecutive channels approximate the one channel. This means in the system disclosed is intimately associated with means for conditioning the system to assign a plurality of the active sources to each of the transmission channels including means responsive to the momentary band width of the signal of each active source, for example during word intervals thereof, for controlling in accordance therewith during such each active-signalinterval the width of the transmission-frequency spectrum assigned to each such active source. This band-width control means essentially assigns to each of at least predetermined ones of the active sources in accordance with the signal band width thereof a width of transmission-frequency spectrum varying with the band width. The last-mentioned means comprises a speech-operated relay 23' included in the second or higher-order station S2 and energized in conventional manner by the speech signal translated by the band-pass selector iii of the first or lower-order station S1 and a speechaaesnoi operated relay 24 also in the station S: and energized in conventional manner by the signal translated by the band-pass selector M of the station S1. Since the station S1 is considered the lowest-order station of the system, no cor responding relays are included in this station. The relays 23' and 24' of station S2 are components of a frequency control system in this station which includes a modulator 25' having an input circuit coupled to an output circuit of the oscillator 20' and also having an input circuit coupled to an output circuit of an oscillator 26' having a frequency of 30 kilocycles. The output circuit of the modulator 25' is coupled through a band-pass selector 21' to an input circuit of a modulator 28', to another input circuit of which is also coupled an output circuit of the oscillator as. The selector 2'5 translates a band of frequencies equal to the sum of any frequency which the oscillator 2@ may have plus 30 kilocycles which is the frequency of the oscillator it. The output circuit of the modulator 28' is coupled to the input circuit of each of three frequency discriminators 29', so and 3!. These discriminators may, for example, be of the type shown in Fig. 52 and described on pages 585-6 of Radio Engineers Handboo by Terman, published by McGraw-Hill Book Company, Inc., of New York, New York (first edition, 1943). The discriminator 29' which is centered on or resonant at a frequency of 30 kilocycles, has an output circuit coupled to a normally closed contact 32' of the relay 23'. The discriminator 30', which is centered on or resonant at a frequency of 28.5 kilocycles, has an output circuit coupled to a normally closed contact 33' of the relay 24. The discriminator 3|, which is centered on or resonant at a frequency of 27 kilocycles, is coupled to a normally open contact 34' of the relay 24'. The movable contact 35 of the relay 24' is coupled to a normally open contact 36' of the relay 23 while the movable contact 3'! of the latter relay is coupled to the input circuit of a frequency-control unit 38. The out put circuit of the latter unit is coupled to a frequency control circuit of the oscillator 20" to control the frequency of the latter.

Each of the transmitting stations includes means for transmitting either-one or two control tones, depending upon the band width of its signal, when the station becomes active, these control tones being utilized to effect the assignment of a corresponding receiving station to the same transmission channel to which the transmitting station is assigned. This means in station S1 comprises a speech-operated relay 39 which is energized by the band-pass selector l3 and which is effective to couple a control-signal generator 40 to a control channel of the transmission line H. The last-mentioned means also includes a speechoperated relay 4! which is energized by the bandpass selector l4 and which is effective to couple a control-signal generator E2 to the control channel of the transmission line ll. Each of the control-signal generators has an individual frequency different than that of any other. For example, the generator 40 has a frequency of 9.9

kilocycles and the generator 42 a frequency of 9.8 kilocycles. In the transmitting station S2, the control signal generator 40 has a frequency of 9.7 kilocycles and generator 42 a frequency of .6 kilocycles. Similarly, in the transmitting station $3, the control signal generator 40" has a frequency 9.5 kilocycles and the generator 42" a frequency of 9.4 kilocycles.

Considering now the operation of the transmitter system just described, assume at the outset that a signal having a frequency band width of to 1.5 kilocycles is developed at a given moment only by the source iii of the transmitting station 31. This signal has no frequency components lyin within the pass band of the selector It so that all of the signal will be translated by the selector IE to the modulator IS. The signal here modulates the oscillations applied to the modulator l from the Oscillator i6 and the upper-sideband modulation components, extending from 92 to 93.5 kilocycles, is selected by the selector l7 and applied to the modulator H8. The signal thus eflectively heterodyned to a higher frequency by the operation of the units I 5, l6 and I7 modulates in modulator It? the oscillations supplied thereto from the oscillator 20. The lower-sideband modulation components, extending from 10 to 11.5 kilocycles, are selected by the selector l9 andapplied to the first transmission channel of the transmission line II, it being assumed that each transmission channel of the latter has a frequency band width of 3 kilocycles and that the channels are considered in order from the lowestfrequency channel to the highest-frequency channel. The speech signal of the source i 0 upon being translated through the selector I8 energizes the relay 39, thus to apply a 9.9 kilocycle control signal to the control-s gnal channel of the transmission line H, and likewise energizes the relay 23 which operates to disconnect the discriminator 29' and connect the discriminator 30 to the frequency-control unit 38. The frequencycontrol unit 38' thereupon effects a decrease of the frequency of the oscillator 20' from 82 kilocycles to 80.5 kilocycles. This frequency-control action will now be considered in greater detail with reference to the transmitting station S2, but it will be understood that the manner of frequency control in this station is representative of that in other stations having such frequency control.

In the transmitting station S2, the outputs oi the oscillators 2i! and 28' are applied to the modulator 25. Of the modulation components developed by the latter, those in the frequency range from 109 to 112 kilocycles are translated by the selector 2? to the modulator 28'. There is also applied to the latter modulator from the oscillator 29 of station S1 oscillations having a frequency of 82 kilocycles. The modulation components developed in the output circuit of the modulator 23' are applied to each of the discriminators 2s, 3d, and 8!. Any one of these discriminators may be coupled to the input circuit of the frequency-control unit 38' depending upon the state of energization of the relays 23 and 241. When the discriminator 29' is coupled to the unit 38', the modulation components translated by the discriminator so control the operation of the frequency-control unit 38, in a manner well understood in the art, that the latter unit maintains the frequency of the oscillator 20' at 82 kilocycles which is the same frequency as that of the oscillater 26 in the transmitting station S1. When the discriminator 38' is coupled to the frequencycontrol unit 38', the frequency of the oscillator as is maintained 1.5 kilocycles below that of the oscillator 20. In like manner, the discriminator 3 I serves to maintain the frequency of the oscillator 2b 3 kilocycles below that of the oscillator 26. It may be noted at this point that in the transmitting station Sa, its modulator 28" is supplied with oscillations from the oscillator 20' of the transmitting station S2. Thus, the discriminators 29", 36" andtll" of station S: serve to maintain its oscillator 2" either at the same frequency as the oscillator 2', 1.5 kilocycles below the frequency of the latter, or 3 kilocycle below the frequency of oscillator 20 depending upon which of the discriminators 29", 30" or 3 l is coupled by selective operation of the relays 23" and 2t" to the frequency-control unit 38".

Under the conditions hereinbefore assumed, the effect of the speech signal translated only through the selector l3 of station S1 is to cause the oscillator 20 of station S2 and the oscillator 20" of station S3 each to have a frequency of 80.5 kilocycles. Hence, if a speech signal having a frequency band width 1.5 kilocycles wide is developed in either (but not simultaneously in both) the transmitting stations S2 or S3, it is translated to the transmission line H in the manner of the speech signal of station S1 previously described and it will be readily apparent that the signal of station S2 or S: occupies the upper half 11.5 to 13 kilocycles of the first transmission channel of line II. If the band width of the speech signal originating in station S2 or S3 is 3 kilocycl es wide, it will now be readily apparent that the portion 1.5 to 3.0 kilocycles of the signal is translated by the lower half 13 to 14.5 kilocycles of the second transmission channel of the transmission line H.

Assume not; that the band width of the speech signal of the source It of station S1, increases to 3 kilocycles. The portion of this signal from 1.5 to 3 kilocycles is now translated by the selector 14 so that the signal when translated to the transmission line i l occupies the entire first transmission channel from 10 to 13 kilocycles. The translation of the signal frequency components by the selector i l efiects the energization of the relay M, which applies a control signal of 9.8 kilocycles to the control-signal channel of the transmission line i i, and also effects the energization of the relay 243' of station S2 thus to disconnect the discriminator 3d and connect the discriminator 3i of the latter station to its frequency-control unit 38'. This has the efiect of decreasing the frequency of the oscillator 2b in station S2 and oi oscillator 28" in station S3 to '79 kilocycles so that the speech signal of either of these stations occupies the lower half or all of the second transmission channel of the transmission line it. It will be apparent that as soon as the band width of the speech signal of source it decreases below 1.5 kilocycles, the relays ll of station S1 and 2d of station S2 become deenergized, thus removing the 9.8 kilocycles control signal from the controlsignal channel of transmission line H and increasing the frequency of the oscillator 2d of station S2 and the oscillator 20" of station S3 to 80.5 kilocycles. Further, as soon as the signal of station S1 disappears entirely, the relays it and 23 become deenergized, thus to remove the 9.9 kilocycle control signal from the control-signal channel and to increase the frequency of the oscillator to of station S2 and the oscillator 20" of station S3 to 82 kilocycles with the result that a signal appearing in either of the latter stations is transmitted over the first transmission channel of the transmission line i i.

This interrelated operation of the transmitting stations, described particularly with relation to stations S1 and S2, applies to all of the stations so that the effect of a speech signal appearing in a lower-order station or source is to shift the speech signals of higher-order stations or sources to higher-order transmission channels of the transmission line I i. It should be remembered that this assignment and reassignment of the active signal sources to available'transmission channels occurs continually and preferably at word intervals. The several speech-operated relays of the system preferably have sufficient hangover time that weak endings of word syllables ar not lost by a premature opening of a relay.

While Fig. 2 shows the circuit arrangement of only three of the transmitting stations S1, S2 and S3 of the Fig. 1 system, it will be understood that the remaining stations S4 and S5 of Fig. l have similar circuit arrangements and that the stationsSs to S10, inclusive, of the second group of transmitting stations also have a similar circuit arrangement and arrangement of elements with the exception that the frequency control of these latter stations is 50 arranged that all of these stations are temporarily assigned the highest-order transmission channel of the transmission lin H and lower-order active sources efiect the reassignment of higher-order sources in order to individual lower-order transmission channels as previously described.

Fig. 3 is a circuit diagram, partly schematic, of several receiving stations R1, R2 and R3 of a signal transmission system embodying the present invention and adapted for operation with the three transmitting stations of Fig. 2 hereinbefore described. These receivin stations include in large part essentially the same circuit elements so that elements of stations R2 and R3 which correspond to the same elements of station R1 are designated by similar reference numerals primed and double primed, respectively. The oscillator frequencies and band-pass selector band widths of the several receiving stations are indicated in the drawings, the indicated values being those suitable for operation with the stations S1, S2 and S3 of Fig. 2. Station R1 includes a modulator 58 having an input circuit coupled through normally open contacts 59 of a relay 52 to the transmission line i l which includes the several transmission channels. The output circuit of an oscillator 53 is also coupled to an input circuit of the modulator 50 and the output circuit of the latter unit is coupled, in the order named, to an adjustable band-pass selector 5:3, a modulator 55, a band-pass selector 56, and a speech-output translating system 5?. The output circuit of an oscillator 59 is coupled to an input circuit of the modulator 55. A control-signal selector 419 is also coupled to the transmission line i l and has a plurality of output circuits each tuned to select an individual one of the control signals transmitted through the control channel of the transmission line i i from the transmitting stations. These output circuits are coupled to the relays 52, 52', 52", and similar relays in the remaining receiving stations, not shown, and are also coupled to a relay 59 of station R1, a relay 59' of station R2,

,and a relay 59 of station R3 and corresponding relays of the remaining receiving stations, not shown.

The receiving stations R2 and R3 include automatic frequency-control systems similar to those of corresponding transmitting stations. This frequency-control system in station R2 comprises a modulator 69' having an input circuit coupled to the output circuit of the oscillator 53 and the output circuit of an oscillator iii. The output circuit of the modulator 69' is coupled through a band-pass selector 62' to an input circuit of a modulator 63'. The output circuit of the oscillator 53 of station R1 is also coupled to an input circuit of the modulator 63' of station R2 and the output circuit of the latter unit is coupled to each of three discriminators 55, 65', and 5G. The output circuit of the discriminator 64 is coupled to a frequency-control unit 61' through a normally closed contact 53 of the relay 52 of station R1. The discriminator 65 is coupled to the unit 6'! through a normally closed contact 59 of the relay 59 of station R1 and through a normally open contact 12 of relay 52 of the latter station. The discriminator 66 is coupled to the frequency-control unit 61 through a normally open contact 10 of the relay 59 and a normally open contact 12 of relay 52. A movable contact H, associated with contacts 69 and 10 of relay 59 is coupled to a normally open contact 12 of relay 52 and the associated movable contact 13 of this relay is connected to the input circuit of the frequency-control unit 61' of station R2.

The relay 59, station R1, has a second pair of fixed contacts 14, 15 and a movable contact 15 which are coupled to a control circuit of the selector 54 to control the pass band of the latter unit.

The operation of the receiving system just described is quite similar to that of the transmitting system previously described except that in the receiving system the control tone received by the selector 49 effect the assignment of the receiving stations to transmitting channels of the transmission line H in such manner that each receiving station is assigned at any given moment to the same channel as its corresponding transmitting station. When no control signals are received by the selector 49, all of the relays 52, 52', 52", 59, etc., are deenergized so that all the receiving stations are disconnected from the transmission line H.

Assume that a control signal of 9.9 kilocycles is received by the selector 49 thus to indicate that the transmitting station S1, which corresponds to the receiving station R1, is transmitting a signal having a band width not exceeding 1.5 kilocycles. This control signal is selected and applied to the relay 52 of station R1. The relay therefrom closes its contacts 5! and couples the input circuit of the modulator 50 to the transmission line I l. The signal received by the modulator 5il modulates the oscillations applied to the latter unit from the oscillator 53 and the upper-sideband modulation components lying within the band of 92 to 95 kilocycles are selected by the selector 541 and applied to the modulator 55. 0scillations from the oscillator 59 are also applied to the modulator 55 and the lower-sideband components of the modulated wave signal developed in the output circuit of the modulator 55, these components comprising the original speech signal of the signal source ill at the transmitting station, are selected by the selector 55 and applied to the speech-output translating system 57 for utilization.

The energization of the relay 52 of station R1 under the assumed conditions causes the input circuit of the frequency-control unit 61' of station R2 to be transferred from the discriminator 66 to the discriminator 65, thus to effect a 1.5 kilocycle decrease of the frequency of the oscillator 53' of station R2 and a similar decrease of the frequency of oscillator 53" of station Ra.

Assume now that a 9.8 kilocycle control signal is simultaneously received by the selector 49, thus to indicate that the band width of the speech signal has increased beyond 1.5 kilocycles, The relay 59 is now energized and the connection of the input circuit of the frequency-control unit 51' is transferred to the discriminator 66' to effect a assaooi R; by an additional 1.5 kilocycles so that the frequencies of these oscillators are now 79 kilocycles. This efiects an assignment of receiving stations R2 and R3 to th next higher-order transmission channel of the transmission line H so that these stations are now conditioned to receive a signal transmitted by their corresponding transmitting station which, as previously described in connection with Fig. 2, are themselves assigned to the next higher-order transmission channel.

Should a 9.7 kilocycle control signal be also received by a selector t9, thus to indicate that the transmitting station S2 was transmitting a signal having a band width not exceeding 1.5 kilocycles, the relay 52' of receiving station R2 would be energized to couple the modulator 50' to the transmission line I! thus to receive the transmitted signal of station S2. 52' frequency-control unit 67" of station R3 to its discriminator 65" thus further to decrease by 1.5 kilocycles the frequency of its oscillator 53". The control signals received by the selector Q9 consequently are effective to assign to each of the receiving stations corresponding to an active one of the transmitting stations the channel and band width assigned to the corresponding transmitting station so that the selector Q9 and associated relays comprise means for effecting this operation. The assignment and reassignment of the receiving stations to the several transmission channels occur continuously and at word intervals in coordinated relation with the assignment and reassignment in similar manner of the transmitting stations.

While only three receiving stations R1, R2 and R3 are shown in Fig. 3, it will be understood that a signal transmission system embodying the invention normally will include a larger number of stations, as indicated in Fig. l, and that the automatic frequency control of the receiving stations will be arranged to effect the assignment of additional stations to the available transmission channels in the manner previously described in relation to Fig. 1.

While the'signal-translating system has been described as of the type which assigns at any given moment to an active station a transmission channel of band width only sufficiently wide to accommodate the speech-frequency components of the speech signal thereof, it will be understood that this is by way of illustration only and that the system may be somewhat simplified by omission therefrom of the band Width assignment feature. Where this is done, an active signal source is simply assigned to an entire transmission channel without regard to the frequency composition of the speech signal of such active source.

While there has been described What is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, means coupling said sources to said channels The operation of the relay also would transfer the input circuit of the i for temporarily assign=- ing all of said sources when inactive to one of said channels, and means responsive to a predetermined operating characteristic of active ones of said sources for controlling said couplingmeans to maintain the lowest-order active source assigned to said one channel and higher-order active sources assigned in order individually to consecutive channels proximate to said one channel.

2. A signal-transmission system comprising, a plurality of speech-signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, means couling said sources to said channels for temporarily assigning all of said sources when inactive to one of said channels, and means responsive to a predetermined operating characteristic of active ones of said sources during word intervals thereof for controlling said coupling means to maintain the lowest-order active source assigned to said one channel and higher-order active sources assigned in order individually to consecutive channels proximate to said one channel.

3. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a, plurality of transmission channels, means coupling said sources to said channels for temporarily assigning all of said sources when inactive to one of said channels, and means responsive to the signal of active ones of said sources for controlling said coupling means to maintain the lowest-order active source assigned to said one channel and higher-order active sources assigned in order individually to consecutive channels proximate to said one channel.

4. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, means coupling said sources to said channels for temporarily assigning all of said sources when inactive t one of said channels, and means responsive to a prede termined operating characteristic of active ones of said sources for controlling said coupling means to maintain from moment to moment the lowest-order active source assigned to said one channel and higher-order active sources assigned in order individually to consecutive channels proximate to said one channel.

5. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels which may be considered as arranged in consecutive order, means coupling said sources to said channels fortempo rarily assigning all of said sources when inactive to an end one of said channels, and means responsive to a predetermined'operating characteristic of active ones of said sources for controlling said coupling means to maintain the lowestorder active source assigned to said one channel and higher-order active sources assignedin order individually to consecutive channels proximate to said one channel. I

6. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, means so coupling said sources to said channels that at least a predetermined number of lower-order sources are assignable to individual channels but each of said sources may temporarily be assigned when inactive to unused channels assignable to lowerorder sources, and means responsive to a predetermined operating characteristic of active ones of said sources for controlling said coupling mean to maintain the lowest-order active source pearing in any single active one of said sources to an end channel of said frequency spectrum, and means responsive to a predetermined operating characteristic of active ones of said sources for controlling said heterodyne means to heterodyne the signal of the lowest-order active source to said end channel and to heterodyne the signals of higher-order active sources individually to consecutive channels proximate to said end channel.

8. A signal-transmission system comprising, a plurality of signal sources, a plurality of transmission channels, means responsive to a predetermined operating characteristic of active ones of said sources for coupling said active sources during the active signal interval of each selectively to said channels, and means for conditioning said system to assign a plurality of said active sources to each said channel including means responsive to the momentary band width of the signal of each said active source for controlling in accordance therewith during said each active signal interval the width of the transmission frequency spectrum assigned to each said active source.

9. A signal-transmission system comprising, a plurality of signal sources, a plurality of transmission channels, means responsive to a predetermined operating characteristic of active ones of said sources for coupling said active sources during the active signal interval of each selectitvely to said channels, and means for conditioning said system to assign a plurality of said active sources to each said channel including means responsive to the momentary band width of the signal of each said active source for controlling in accordance therewith during said each active signal interval the width of the transmission frequency spectrum assigned to each said active source.

10. A signal-transmission system comprising, a plurality of speech-signal sources, a plurality of transmission channels, means responsive to a predetermined operating characteristic of active ones of said sources for coupling said active sources during the active signal interval of each selectively to said channels, and means for conditioning said system to assign a plurality of said active sources to each said channel including means responsive to the band width of the signal of each said active source during Word intervals thereof for controlling in accordance therewith during said each active signal interval the width of the transmission frequency spectrum assigned to each said active source.

11. A signal-transmission system comprising, a plurality of signal sources, a plurality of transmission crannels, means responsive to a predetermined operating characteristic of active ones of said sources for coupling said active sources during the active signal interval of each selecaasaoor tively to said channels, and means for conditioning said system to assign a plurality of said active sources to each said channel including means for allotting to each of at least predetermined ones of said active sources in accordance with the signal band width thereof a width of transmission frequency spectrum varying with said band width. 5

12. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, means for coupling active ones of said sources selectivel to said channels always with the lowest-order active source coupled to a predetermined channel and higherorder active sources coupled in consecutive order to consecutive channelsproximate to said Predetermined channel, and means responsive to the band width of the signalof each said active source for controlling in accordance therewith the width of the transmission frequency spectrum assigned to said each active source.

13. A signal-transmission system comprising, a plurality of signal sources which may be considered as arranged in consecutive order, a plurality of transmission channels, means for coupling active ones of said sources selectively to said channels always with the lowest-order active source coupled to a predetermined channel and higherorder active sources coupled in consecutive order to consecutive channels proximate to said predetermined channel, and means responsive to the momentary band width of the signal of each said active source for controlling in accordance therewith the width of the transmission frequency spectrum assigned to said each active source.

14. A signal-transmission system comprising, a plurality of signal sources and corresponding signal-receiving stations which may be considered as arranged in consecutive order, a plurality of transmission channels, means responsive to a predetermined operating characteristic of active ones of said sources for assigning the lowest-order active source to one of said channels and higherorder active sources individuall to consecutive channels proximate to said one channel, and means responsive to a control characteristic of active ones of said sources for assigning to each of said stations corresponding to an active one of said sources the channel assigned to its corresponding active source.

15. A signal-transmission system comprising, a pluralit of speech-signal sources and corresponding speech-signal receiving stations which may he considered as arranged in consecutive order, a plurality of transmission channels, means responsive to a predetermined operating characteristic of active ones of said sources for assigning during word intervals thereof the lowest-order active source to one of said channels and higher-order active sources individuall to consecutive channels proximate to said one channel, and means responsive to a control characteristic of active ones of said sources for assigning to each of said stations corresponding to an active one or said sources the channel assigned to its corresponding active source.

16. A signal-transmission system comprising, a plurality of signal sources and corresponding signal-receiving stations, a plurality of transmission channels, means responsive to a predetermined operating characteristic of active ones of said sources for coupling said active sources during the active signal interval of each selectivel to said channels, means for conditioning said sysasaaooi tem to assign a plurality of said active sources to each said channel including means responsive to the bandwidth of the signal of each said active source for controlling in accordance therewith during said each active signal interval the width of the transmission frequency spectrum assigned to each said active source, and meansresponsive to a control characteristic of active ones of said sources for assigning to each of said stations corresponding to an active one of said 10 sources the channel and band width assigned to ones of said sources selectively to said channels always with the lowest-order active source coupled to a predetermined channel and higher-order active sources coupled in consecutive order to consecutive channels proximate to said predetermined channel, means responsive to the band width'of the signal of each said active source for controlling in accordance therewith the width of the transmission frequency spectrum assigned to said each active source, and means responsive to a control characteristic of active ones of said sources for assigning to each of said stations corresponding to an active one of said sources the channel and band width assigned to its corresponding active source.

' ARTHUR V. LOUQ 

